Continuously pumped q-switched arrangement including an n{11 :yaig laser element

ABSTRACT

Q-switching of a laser is accomplished by controlling the state of an intracavity acousto-optic switch that comprises a piezoelectric transducer bonded to a low-optical-loss ultrasonic propagation medium. Energization of the transducer causes a traveling acoustic diffraction grating to propagate through the medium. Illustratively, the direction of propagation of the grating is perpendicular to the direction in which light travels in the laser. The interaction between the light and the acoustic grating gives rise to diffraction losses that prevent the laser from oscillating. In response to a momentary deenergization of the transducer, the laser cavity is restored to a high-Q oscillatory condition during which energy stored in the system during the nonoscillatory state is suddenly released. During each such deenergization period, a high-amplitude output pulse of coherent radiation is obtained.

United States Patent Inventors Appl. No.

Filed Patented Assignee Joseph E. Geusic Berkeley Heights;

Michael A. Karr, III, Murray Hill, both of 805,202 Mar. 7, 1969 Oct. 12,1971 Bell Telephone Laboratories, Incorporated Murray Hill, NJ.

US. Cl

Int. Cl

Field of Search PUMP SOURCE PARTIALLY -TRANSM|SSIVE E ENT LASER LEMENT[56] References Cited UNITED STATES PATENTS 3,297,876 1/1967 DeMaria331/945 Primary ExaminerWilliam L. Sikes Attorneys-R. J. Guenther andKenneth B. Hamlin ABSTRACT: Q-switching of a laser is accomplished bycontrolling the state of an intracavity acousto-optic switch thatcomprises a piezoelectric transducer bonded to a low-opticallossultrasonic propagation medium. Energization of the transducer causes atraveling acoustic diffraction grating to propagate through the medium.lllustratively, the direction of propagation of the grating isperpendicular to the direction in which light travels in the laser. Theinteraction between the light and the acoustic grating gives rise todiffraction losses that prevent the laser from oscillating. In responseto a momentary deenergization of the transducer, the laser cavity isrestored to a high-Q oscillatory condition during which energy stored inthe system during the nonoscillatory state is suddenly released. Duringeach such deenergization period, a high-amplitude output pulse ofcoherent radiation is obtained.

CONTROL ccr a .1 S I L SIGNAL TRANSDUCER E G HlGHLY REFLECTIVE K28ELEMENT CONTINUOUSLY PUMPED Q-SWlTCI-IED INCLUDING AN NDzYAIG LASERELEMENT BACKGROUND OF THE INVENTION 1. Field of the Invention Thisinvention relates to signal translation and more particularly to aQ-switched laser arrangement.

2. Description of the Prior Art In 1961 Hellwarth proposed thegeneration of very intense and short bursts of radiation from laserdevices (see Advances in Quantum Electronics, ed. by J. R. Singer,Columbia University Press, 1961, pp. 334-341). His proposal was based onthe sudden application of high regenerative feedback to an excited lasersystem. This technique, now called laser Q- switching, was subsequentlydescribed by McClung and Hellwarth as being achieved with a Kerr cell(J. Appl. Phys., Vol. 33, pp. 828-829, March 1962), by Collins andKisliuk with a rotating disc. (J. Appl. Phys., Vol. 33, pp. 2009-2011,June 1962) and by DeMaria, Gagosz and Barnard with anultrasonic-refraction shutter (J. Appl. Phys., Vol. 34, pp. 453-456,March 1963). Rotating mirrors and prisms, Pockels cells and saturableabsorbers have also been suggested for use in Q-switched laserarrangements (see P. P. Sorokin, J. J. Luzzi, J. R. Lankard and G. D.Pettit, IBM J. Research and Develop., Vol. 8, pp. 182-194, Apr. 1964; P.Kafalas, J. 1. Masters and E. M. E. Murray, J. Appl. Phys., Vol. 35, pp.2349-2350, Aug. 1964; and L. M. Frantz and J. S. Nodvik, J. Appl. Phys.,Vol. 34, pp. 2346-2349, Nov. 1963).

The neodymium-containing yttrium aluminum garnet (NdzYalG) laserdeveloped by Geusic and Van Uitert (see US. Pat. No. 3,252,103, issuedMay 17, 1966) is an advantageous solid-state laser capable of continuousroom-temperature operation. It has been demonstrated that this type oflaser is capable of Q-switched operation. Specifically, repetitivelyQ-switched NdzYAlG lasers have been operated using both rotatingreflectors and electro-optic modulators (see J. E. Geusic, M. L. Henseland R. G. Smith, Appl. Phys. Lett., 6, pp. 175-177, May 1, 1965; and R.G. Smith and M. F. Galvin, IEEE J. Quan. Elec., QE-3, pp. 406-414, Oct.1967). However, both of these modulating techniques exhibit somepractical disadvantages which stem from various mechanical problemsinherent in rotating reflectors and the unavailability of suitablelow-loss electro-optic crystals.

SUMMARY OF THE INVENTION Accordingly, an object of the present inventionis an improved laser arrangement.

More specifically, an object of this invention is an improved Q-switchedlaser arrangement of the NdzYAlG type.

Another object of the present invention is a Q-switched NdzYAlG laserarrangement well suited for use in various micromachining applicationsof practical interest.

A still further object of this invention is a Q-switched Nd:YA1G laserarrangement capable of generating highrepetition-rate high-peak-powerpulses exhibiting an advantageous pulse-to-pulse amplitude stability.

Briefly stated, these and other objects of the present invention arerealized in a specific illustrative embodiment thereof which comprises acontinuously pumped NdzYAlG laser atrangement that includes anintracavity acousto-optic modulator. The modulator comprises apiezoelectric transducer bonded to a low optical loss ultrasonicpropagation medium.

The transducer is electrically driven to launch a traveling acousticdiffraction grating in the ultrasonic medium. Illustratively, the axisalong which the optical beam of the arrangement is designed to travel isperpendicular to the direction in which acoustic waves are propagated inthe medium. Sufflcient electrical power is applied to the transducer tocause the diffraction losses per pass in the arrangement to exceed thecharacteristic gain per pass. Thus, during the time in which a travelingacoustic wave exists in the ultrasonic medium, no laser oscillationsoccur. During this time, energy is accumulated in the upper level of thelaser transition of the Nd:YA1G systeml Q-switching is accomplished bypulsing of the electrical signals applied to the transducer. Thisrestores the laser to a high-Q oscillatory condition. During thisrestoration period, the stored energy of the system is suddenly releasedwhereby a high-amplitude pulse of coherent radiation is obtainedtherefrom.

It is a feature of the present invention that a Q-switched laserarrangement include an ultrasonic propagation medium for propagating atraveling acoustic diffraction grating that introduces sufficient lossesin the arrangement to prevent oscillations from occurring therein.

It is another feature of this invention that circuitry be provided tomomentarily terminate the traveling grating thereby to restore thearrangement to a high-Q oscillatory condition and to induce ahigh-amplitude pulse to occur during the termination period.

BRIEF DESCRIPTION OF THE DRAWING A complete understanding of the presentinvention and of the above objects, features and advantages thereof maybe gained from a consideration of the following detailed description ofa specific illustrative embodiment thereof presented herein below inconnection with the accompanying drawing, in which:

FIG. 1 depicts a specific illustrative embodiment made in accordancewith the principles of the present invention; and

FIG. 2 depicts several waveforms that are helpful in understanding themode of operation of the FIG. 1 arrangement.

DETAILED DESCRIPTION The specific illustrative laser arrangement shownin FIG. 1 includes a conventional laser element 10 which may, forexample, comprise a neodymium-containing yttrium aluminum garnet (NdzYAlG) rod of the type described in the aforecited Geusic-Van Uitert patent.Illustratively, the element 10 is a cylindrical rod whose main axis iscoincident with the longitudinal axis 11 (dot-dash line) of the laserarrangement. Typically the ends of the laser element 10 are flat,parallel and coated to be antireflecting at a wavelength of 1.06microns. Continuous pumping of the element 10 to achieve an output at1.06 microns is achieved by means of a conventional pump source 12 whoseradiant output, directed at the element 10, is represented by dashedarrows 14.

The laser element 10 of FIG. 1 is contained in a conventional cavitydefined, for example, by a curved member 16 and a highly reflectiveelement 18. The member 16 has a partially transmissive element 16acoated, deposited, adhered or otherwise suitably disposed on the concavesurface thereof. Illustratively, the element is selected to transmit toan output element 20 about 1.6 percent of the 1.06-micron radiation thatimpinges on the concave side of the composite 16-l6a structure. Theelement 20 may, for example, comprise a work piece such as a thin-filmresistor whose characteristics are to be precisely controlled bylaser-micromachining techniques that involve selective material removal.For such purposes either the laser arrangement or the work piece or bothmay be moved by conventional micropositioning apparatus (not shown).

The element 18 shown in FIG. 1 comprises, for example, a conventionalhighly reflective coating which is designed to reflect approximately99.95 percent of the 1.06-micron radiation incident on the left sidethereof. Advantageously, the element I8 is coated on a supporting member22 that comprises an ultrasonic-propagation medium which is selected,position, proportioned and driven in accordance with the principles ofthe present invention.

In accordance with this invention, the ultrasonic member 22advantageously comprises a fused silica block. In its quiescent orundriven state such a block exhibits a low optical loss to 1.06-micronradiation propagated therethrough along the axis 11. Accordingly, if themember 22 is not activated, the depicted arrangement is well suited forconventional continuouswave operation.

Activation of the ultrasonic member 22 in accordance with the principlesof the present invention is achieved by means of a conventionalpiezoelectric transducer 24 which, for example, is bonded to the topsurface of the member 22. The transducer 24 is driven by aradiofrequency sine-wave signal supplied by a generator 26. Whether ornot the continuous output of the generator 26 is actually applied to thetransducer 24 is determined by the state of a conventional switch 28whose condition (open or closed) is in turn controlled by a conventionalcircuit 30.

Illustratively, the bottom surface of the ultrasonic member 22 isbeveled to prevent the formation of acoustic standing waves. (Inaddition, it is advantageous to place a suitable energyabsorbing elementin contact with the sides and bottom of the member 22. In particular, ithas been determined that a fiber glass tape wrapping about the sides andbottom of the member 22 aids in preventing the formation of standingwaves therein.) Accordingly, the generator 26 and the transducer 24 areeffective, when the switch 28 is controlled by the circuit 30 to be inits closed or transmitting state, to launch a progressive or travelingacoustic wave in the member 22. By selecting the output frequency of thegenerator 26 such that several wavelengths of the traveling acousticwave are encompassed within the width of the light beam generated inthedepicted laser arrangement, the launched wave acts as a travelingacoustic-diffraction grating with respect to the beam. Illustratively,the grating is launched to travel in a direction perpendicular to theaxis 11 along which the light beam is designed to propagate.

The interaction between the aforementioned light beam and the travelingacoustic grating gives rise to diffraction losses. (For a discussion ofthis type of interaction, see Principles of Optics, by M. Born and E.Wolf, Pergamon Press, NY. 1964, pp. 593-610.) By launching relativelylarge-amplitude acoustic waves in the ultrasonic member 22, sufficientspatial dispersion of the light beam occurs that the loss per pass dueto diffraction in the laser cavity exceeds the characteristic gain perpass of the laser arrangement. In this way the member 22 is effective,when suitably activated, to prevent the occurrence of oscillations inthe arrangement. During this nonoscillatory condition, no output signalis directed at the element 20. However, during this condition, thesource 12 continues to pump the laser element and, as a result, energyis accumulated in the upper level of the laser transition of the NdaYAlGsystem.

The overall mode of operation of the FIG. 1 arrangement can be bestunderstood by reference to the waveforms of FIG. 2. At time 2, shown inFIG. 2, the signal generator 26 of FIG. 1 is assumed to commenceproviding an output signal. Between times 1, and the switch 28 isassumed to be in its blocking state, and hence no driving signals areapplied to the transducer 24. During that interval, the depictedarrangement oscillates in its conventional continuous-wave manner and,as shown in FIG. 2, a relatively low-amplitude output signal isdelivered to the element 20.

At time the circuit 30 controls the switch 28 to allow the output of thegenerator 26 to be applied to the transducer 24. Accordingly, at thattime a traveling acoustic diffraction grating is launched in theultrasonic member 22 and, as described above, the laser arrangement isthereby rendered nonoscillatory. The absence of an output signal fromthe arrangement during the interval t through is represented in thebottom waveform of FIG. 2.

At time t;, shown in FIG. 2, the circuit 30 of FIG. 1 opens the switch28 thereby deactivating the transducer 24 and terminating theaforementioned traveling acoustic diffraction grating. In the absence ofthe aforementioned diffraction losses, the laser arrangement is returnedto a high-Q oscillatory condition. During this condition, theaforementioned energy accumulated in the upper level of the lasertransition is suddenly released. This sudden release causes ahigh-amplitude pulse 35 (FIG. 2) of coherent radiation to be directed atthe output element 20.

After the high-amplitude output pulse 35 has terminated (at time andbefore the laser arrangement commences to oscillate in itscontinuous-wave mode of operation, the drive to the transducer 24 isresumed, at time In the interval 1,, through t, a the aforementioneddiffraction losses introduced by the ultrasonic member 22 are againeffective to maintain the arrangement in its nonoscillatory condition.

In one specific illustrative Q-switched NdzYAlG embodiment of theprinciples of the present invention, the generator 26 provided aI00-volt RMS signal at 40 MHz. The transducer 24 comprised a 0.0025 inchX 0.125 inch X 2.00 inch X-cut natural quartz element. This element wasbonded to the top surface of an ultrasonic delay member 22 thatcomprised a 0.150 inch X 0.825 inch X 2.00 inch fused silica block whosebottom surface was beveled at 30 to prevent standing acoustic waves.(Advantageously, as indicated above, the sides and bottom of the member22 may also be wrapped with fiberglass tape.) Q-switching of thisparticular embodiment with a pulse-to-pulse stability of better than 1percent was achieved. Output pulses each having a peak power ofapproximately one kilowatt at 1.06 microns were obtained from thearrangement at repetition rates up to about 5,000 pulses per second.Higher repetition rates, up to about 50,000 pulses per second, wereobtained at lower power levels. (By contrast the peak-power outputduring continuous-wave operation was about one watt.) In the Q-switchedmode of operation the output pulse width (measured at the half-powerpoint) was typically about 300 nanoseconds.

It is to be understood that the above-described arrangement is onlyillustrative of the application of the principles of the presentinvention. In accordance with these principles, numerous otherarrangements may be devised by those skilled in the art withoutdeparting from the spirit and scope of the invention. For example,although specific emphasis herein has been directed to a Nd:YAlG lassersystem, it is to be understood that other laser elements havingtransitions characterized by moderately long lifetimes are also suitedfor inclusion in embodiments of the present invention. Moreover, variousknown techniques in the laser art may advantageously be incorporated inembodiments of this invention. Thus, for example, by positioning a Ba;NaNb, 0,; crystal (described by J. E. Gevsic, H. .l. Levinstein, J. .l.Rubin, S. Singh and L. G. Van Uitert, Appl. Phys. Lett. 11, 269, 1967)in the cavity of a Q- switched NdzYAlG laser, the normal 1.06micronoutput thereof is converted to an output having a wavelength of 0.53microns. It has been observed that this conversion in the Q- switchedmode of operation can be achieved with smaller crystals than arerequired in continuous-wave operation.

In addition, it is emphasized that the particular materials specifiedherein for the transducer 24 and the ultrasonic propagation member 22are intended to be illustrative only. Other transducer materials, suchas, for example, lithium niobate, and other ultrasonic-propagationmaterials, such as, for example, lead molybdate, are well suited forinclusion in embodiments of this invention. .ludicious selection ofvarious combinations of available materials for the members 22 and 24can result in substantially reducing the driving power needed to launchthe required traveling acoustic-diffraction grating in the member 22.

For illustrative reasons the direction of propagation of the travelingacoustic-diffraction grating has been described above as beingperpendicular to the direction in which the laser-generated light beampropagates. Alternatively, the arrangement may be structured such thatthe grating and beam are oriented at the Bragg angle with respect toeach other. In this alternative arrangement the driving power requiredto launch the grating is reduced.

What is claimed is:

1. In combination in a Q-switched laser arrangement,

oscillator means, including an NdzYAIG laser element characterized by atransition having a moderately long lifetime, for generating a coherentbeam of radiation and propagating said beam along a main axis of saidarrangement,

said oscillator means further including means of continuously pumpingsaid laser element to achieve an output at [.06 microns,

a fused silica block interposed in the path of said beam for propagatinga traveling acoustic-difiraction grating,

said block including first and second opposed surfaces and beingcharacterized by a low optical loss to 1.06-micron radiation propagatedtherethrough,

one of said opposed surfaces of said block being beveled to prevent theformation in said block of acoustic standing waves,

means coupled to the other one of said opposed surfaces of said blockfor driving said block to launch therein a traveling acoustic gratingthat causes sufficient diffraction losses in said arrangement to exceedthe characteristic gain thereof thereby to maintain said oscillatormeans in a nonoscillatory condition during which energy is accumulatedin the upper level of the laser transition of said NdzYA [6 laserelement,

and means connected to said driving means for momentarily interruptingthe launching of said grating thereby to allow said oscillator means toassume a low-loss oscillatory condition during said interruption and toprovide a high-amplitude output signal derived from the energy stored inthe combination during said nonoscillatory condition.

